For most surgical procedures, it is advantageous for a surgeon to compare intra-operative progress and post-operative results with a pre-operative plan to ensure that surgical objectives are met. In some surgical procedures, particularly those involving orthopedic arthroplasty, relatively small deviations from a pre-operative plan can translate into significant differences in the functionality of the patient's anatomy. For example, in joint replacement surgery on the knee or hip, small changes in the positioning of the prosthetic joint components may result in considerable differences in the patient's posture, gait, and/or range of motion.
During orthopedic procedures involving resurfacing, replacement, or reconstruction of ball-and-socket joints, such as in the hip, surgeons attempt to ascertain differences between the pre-operative joint and the reduced, newly-implanted joint. The surgeon may analyze these differences during the surgery to evaluate the accuracy of the position of the implant and determine whether additional adjustment of the implant is required before finishing the surgery.
There are three commonly-used parameters that are used to quantify differences in prosthetic joint placement: leg length (also called hip length), offset, and anterior/posterior position. Leg length refers to the longitudinal extent of the leg measured along the superior/inferior axis relative to the pelvis. Offset refers to the position of the leg along the medial-lateral axis relative to the pelvis. Anterior/posterior (“AP”) position of the leg, as the name suggests, refers to position of the leg along the anterior/posterior axis with respect to the pelvis.
Early methods for calculating leg length, offset, and anterior/posterior position required the surgeon to use rulers and gauges to perform manual measurements on the hip joint before and after attaching the prosthetic implants. Such measurements, however, are often inaccurate due to the difficulty in performing manual measurements in the surgical environment using conventional rulers and gauges. Further, manual measurements are not easily repeatable or verifiable, and can take a significant amount of time to perform.
Subsequent development of computer-assisted surgical navigation systems enabled real-time tracking of relative positions of bones and other anatomical features in a defined coordinate space. Using these systems, surgeons electronically register the position and orientation of certain anatomical landmarks, such as bones, and track the position and orientation of these landmarks relative to other defined landmarks. Conventional computer-assisted techniques for calculating leg length, offset, and anterior/posterior position in hip replacement procedures involved using these tracking capabilities to electronically measure a pre-operative position of the femur relative to the pelvis in a specific predetermined reference position. After the hip replacement prostheses are implanted and the joint is reduced, the surgeon attempts to manually return the femur according to a surgical plan. Once the surgeon believes he has positioned according to the plan, the surgeon prompts the system to determine changes in leg length, offset, and anterior/posterior position.
Although conventional computer-assisted techniques for measuring hip replacement parameters may increase the speed and precision of the measurements, their accuracy depends largely upon the accuracy with which the surgeon positioned the femur after implantation of the replacement joint. Indeed, if the surgeon fails to return the femur to the precise position and orientation, the hip parameters are subject to considerable error. In some cases, this error may lead the surgeon to conclude that an improperly-aligned prosthetic is within an acceptable threshold of planned surgical goals, and end the surgery. In other cases, an alignment error may lead the surgeon to conclude that a properly-aligned prosthetic deviates unacceptably from the threshold of planned surgical goals, causing the surgeon to make unnecessary adjustments to the position of the prosthetic implant(s) that may lead to a misalignment. In either case, the patient may experience post-operative discomfort or, in the case of a severe misalignment, a dislocation of the prosthetic femoral head from the acetabular cup. Such discomfort and/or dislocation may require corrective surgery, prolonged rehabilitation, and undue expense.
The presently disclosed systems and methods for measuring parameters in joint replacement surgery are directed to overcoming one or more of the problems set forth above and/or other problems in the art.